1. Field of the Invention
The present invention relates to lighting devices for automotive vehicles and the production method thereof.
2. Description of the Related Art
As known in the art, an automotive light is a lighting and/or signaling device of a vehicle including at least one vehicle outer light having a function of lighting and/or signaling to the outside, such as a position light, a direction indicator light, a brake light, a rear fog light, a reversing light, a low beam, a high beam, and the like. The terms “lighting device” and “signaling device” denote an automotive headlight and, in particular, both an automotive signal light and an automotive headlight, also called a projector, as well as further lighting and/or signaling devices for vehicles, such as ceiling lights and the like. The term “headlight” denotes a lighting device for vehicles includes a lighting and/or signaling device of a vehicle that include at least one light having a lighting and/or signaling function.
The lighting and/or signaling device, in its most simple form, includes a container body, a lenticular body, and at least one light source. The lenticular body is placed for closing a mouth of the container body so as to form a housing chamber. The light source is arranged within the housing chamber and can be oriented so as to emit light towards the lenticular body when powered with electricity. Within the housing chamber, the lighting and/or signaling devices may include light guides or further lenticular bodies having optical properties when associated to a light source, such as filters or lenses. In some cases, in order to achieve a particular diffusion of light output by the lighting and/or signaling device, one or more of the lenticular body and the light guide are made with transparent materials molded on embossed surfaces. Embossed surfaces may be front or rear surfaces, or both, of the lenticular body or light emitting surfaces of the light guide.
The embossing of the mold is replicated by a fluid plastic material, which retains the rough structure of the surface after the polymer freezes. The result is that the lenticular body or light guide, the surface of which is structured with a determined roughness, optically diffuses the light that it receives from the source that illuminates it. The technological difficulty associated with the use of this surface embossing molding replication technique is that the quality of the replica is strongly dependent on the retaining pressure, during the freezing step of the molten polymer into the mold cavity. In fact, insufficient pressure applied during compaction of the material leads to volumetric shrinkage resulting from the freezing of the plastic, which makes replication of the embossing inconsistent. This effect is even more pronounced when the lenticular body or the light guide are larger and thicker, and when the injection point is farther away. Moreover, with this technique it is difficult to retain the diffusive optical characteristics of the molding product over time, since the wear of the mold can alter the embossing, and the mold with embossed surface cannot be polished, thus necessitating a much more complex maintenance schedule requiring the re-execution of the mold surface.
In order to overcome this drawback, in some cases it is possible to use inherently opal materials, such as PLEXIGLAS® Satin Ice by Evonik, in which the polymer granule is admixed with microspheres of polymeric material with different refractive index dispersed in the volume of the granule itself. The different refractive index between the master material and diffused microspheres causes the incident light to deviate its direction of rectilinear propagation (typical of the homogeneous medium), thus affecting another microsphere or exiting from the surface of the plastic with an altered direction. This phenomenon causes the opalescence of the base material and, thus, also that of the moulded material.
This type of technique, which employs polymeric microspheres dispersed in a matrix of another transparent polymer with a different refractive index, overcomes the imperfect replication of the embossing described above, ensuring a homogeneous distribution of the opalescence in transmission, which only depends on the concentration of microspheres in the matrix and the thickness of the lenticular body or the size of the light guide (greater thickness=greater opalescence).
Moreover, because the surfaces of the lenticular body or of the light guide can be smooth, since the opalescence characteristic is given by the volume of the material and not by its surface, it is possible to maintain the surfaces of the molds with a simple mechanical polishing process.
Finally, unlike the optical behavior of the homogeneous transparent material with an embossed surface, the opalescent polymer with microspheres diffuses the incident light from its volume and not from its surface, thus giving the illuminated material a fuller, more uniform and more transparent appearance. Specifically, the light is diffused from the inside of the material; not only from its surface.
However, opaline material with dispersed microspheres encounters two obstacles to its introduction in the automotive field. A first obstacle is regulatory in nature: some fundamental and unavoidable standardized acceptance tests of the materials for lighting and signaling on the North American territory (USA and Canada) require that the virgin material (granule) for optical use to have a very high light transmission without diffusion. Inherently, the polymer with diffusing microspheres is specially formulated to diffuse the light is unusable. On the other hand, a transparent material used for molding with embossed mold imprint is transparent and, therefore, acceptable for the North American market. This opens a problem of marketability of vehicles throughout the world, and forces the use of two types of equipment and two different materials; a mold with embossed surfaces and the use of the standard transparent material for the North American market, and a mold with smooth surfaces and the use of the material with microspheres for the other markets. Utilizing two different types of equipment negatively impacts cost, productivity, and product quality.
A second obstacle comes from the fact that different lenticular bodies, or likewise different light guides having a diffusing optical behavior, must be respectively formed with different materials with microspheres. More precisely, a material with microspheres suitable for forming a lenticular body having a first thickness can be obtained using plastic material with a first particle size of microspheres. A material with microspheres suitable for forming a lenticular body having a second thickness can be obtained using plastic material having a second particle size of microspheres. This constitutes a problem for the manufacturers of goods provided with components formed with the material in microspheres, since the choice regarding the particle size of microspheres can be made only in the procurement step of the plastic material.
Disadvantageously, moreover, the usual methods of production allow only materials with microspheres having a homogeneous structure to be obtained. In other words, the density and/or size of the microspheres is substantially constant within all the material with microspheres.